This invention concerns casting slurries, particularly investment casting slurries, and a method for processing mold-forming materials to decrease slurry aging and increase slurry lifetime.
Investment casting is a process for making articles, referred to as castings, from metals and alloys. Castings are formed by pouring molten metals or alloys into molds having an internal cavity shaped in the form of a desired article. The metal or alloy is allowed to solidify inside the mold, and the mold is then separated from the casting. The mold is typically made from refractory materials, such as ceramics.
Investment casting molds can be formed by a dip coating-process in which wax or polymeric xe2x80x9cpatternsxe2x80x9d formed in the shape of a desired article are dipped into a casting slurry. A casting slurry is a system in which small particles, such as small particles of refractory materials, are uniformly dispersed in a liquid. The liquid can be a variety of materials, such as water or an organic material, including but not limited to alcohols. The wet slurry material is allowed to at least partially dry to form a covering over the pattern. The pattern can be repetitively dipped to build up a coating of the desired thickness.
Aqueous slurries for processing ceramics are cheap and relatively environmentally safe. However, aqueous processing of some refractory materials, such as ceramics, is difficult because they dissolve in water. Organic-based slurries also have this problem because such slurries typically include water. It is believed that a common pathway for dissolution of ions from ceramics is the hydration of surface ions. xe2x80x9cHydrationxe2x80x9d means that the ceramic particles react with water to form a chemical bond. The compounds formed by hydration are referred to as hydrates. Surface ions adsorb water, react with the adsorbed water to form hydrates, and then dissolve.
Aging is defined as any change in slurry property versus time. Hydration of refractory powders results in slurry aging by, for example, increasing slurry viscosity. Aging can contribute to lower shell quality in many ways. For example, as ceramic materials dissolve, the dissolved species may substantially change the ionic strength of the solution and consequently agglomerate the particles. This can adversely affect mechanical properties of the constructed shell. Furthermore, if the charge of the species that dissolves is different from the charge of the ceramic particles or other components of the slurry (e.g., the ceramic particles have negative charge, but the dissolving species have positive charge or vice versa), then the dissolving species may preferentially adsorb onto different components of the slurry. This may change the rheological (the deformation and flow of matter) properties of the slurry, as well as other slurry properties of interest. Finally, as ceramic materials react with water, some ions may preferentially dissolve relative to others, which consequently changes the ratio of ions in suspension and solution. This may result in changes in the physical or chemical properties of the ceramic.
The dip-coating process generally requires using large volumes of slurries. It may take weeks or months before the whole quantity of slurry required for the dip-coating process is consumed. Unfortunately, slurry aging typically requires that the slurry be discarded well before it can be used to form casting molds. This is both costly and wasteful.
Powders are substantially free of surface hydroxides immediately after being manufactured at high temperatures. Slurry materials may be exposed to high temperatures during the initial manufacturing processes and during any subsequent fusion or sintering processes. Fusion and sintering are two methods of increasing the particle size of refractory powders. Fusion involves heating a powder above its melting point to produce a liquid. The liquid is cooled, re-crystallized and ground into a more desirable particle size distribution. Sintering involves fusion of fine particles upon heating at temperatures below the complete melting point of the powder. When sintering is completed, the sintered material is ground to a desirable powder size.
However, once the manufactured powders are exposed to ambient water in the atmosphere, hydration begins. Because refractory powders are manufactured in bulk and often are transported over long distances, it is not always practical or cost-efficient to either use or adequately seal the powders immediately after they are manufactured. Moreover, even if the powders are sealed at the factory sufficient to prevent hydration, which generally is not the case, a consumer who does not consume all of the powder once it is unsealed will have to store the unused portion. Unless the powders are (1) used immediately after they are manufactured, or (2) sealed in a water-free container immediately after they are manufactured and subsequently used immediately after unsealing, they will undergo surface hydration. In practice, neither 1 nor 2 are practicable; hence, refractory powders typically are used as hydrates to form casting slurries.
Hydration of refractory materials may be temporarily reduced by the consumer if the consumer undertakes further manufacturing of the powders, for example by sintering or fusing the powders, after they are received from the original manufacturer. However, the resulting powders immediately begin to rehydrate unless steps are taken to prevent hydration.
A number of solutions have been offered to control aging of investment casting slurries. Horton""s U.S. Pat. No. 4,947,927 shows that increasing the pH to above 11 can reduce aging of yttria slurries. This is because yttria dissolution decreases with increasing pH. However, maintaining yttria slurry pH in a production environment above 11 at all times is inconvenient and impractical. Furthermore, compositions exhibit increased toxicity as the pH varies significantly from neutral.
Yasrebi et al.""s U.S. Pat. Nos. 5,407,001 and 5,643,844 teach decreasing the overall dissolution rate of an oxide by doping the oxide with a material having a lower solubility in the slurry medium, typically water, than the oxide. Consequently, slurry aging can be reduced. Coating powder surfaces with a protective layer also can reduce slurry aging. Yasrebi et al. in U.S. Pat. No. 5,624,604 show that adsorption of hydroxylated benzoic acid onto the surface of rare earth oxides reduces their dissolution rate and thereby reduces slurry aging.
Persons skilled in the art of ceramic processing have long sought simple and inexpensive method, to increase the lifetime of casting slurries. Despite the prior inventions directed to this objective, there still is a need for convenient and practical methods for increasing the useful lifetimes of investment casting slurries.
This invention provides a method for increasing the lifetime of a casting slurry. One feature of the invention is processing slurry materials at a first hydration level to produce materials having a second, lower hydration level before they are incorporated into the slurry. This results in a substantial increase in the lifetime of a slurry made using such processed materials compared to slurries made using materials not processed as described herein.
One embodiment of the present method for increasing the lifetime of a casting slurry comprises heat processing at least one slurry material, typically refractory powders which have undergone hydration subsequent to commercial production, for a period of time sufficient to reduce the amount of hydration from a first hydration level to a second hydration level. A slurry is formed using the slurry material at a hydration level which provides an increased slurry lifetime relative to the same material without processing according to the method of the present invention. Slurry formation can be accomplished substantially immediately after processing, or up to about one week after processing, typically less than 24 hours after processing, and even more typically within 2 hours to about 8 hours after processing. If the material is stored under a substantially water-free atmosphere, then the materials can be used later.
The temperature and the time associated with processing the slurry material can be selected based on considering the extent to which slurry lifetime is increased. This can be measured using, for example, isoelectric points. One embodiment of the method comprises heating the slurry material, such as refractory powder, for a period of time sufficient to change the isoelectric point of the material from an initial pH to a second pH, such as where the initial and second isoelectric points differ by at least 0.5 pH unit, even more preferably by greater than at least 1.0 pH unit, and even more preferably by at least 1.5 pH units. Heat processing also can be done within particular temperature ranges, which is best determined with reference to particular materials. For yttria and alumina, working embodiments of the method have heated such materials at temperatures of from about 500xc2x0 F. (260xc2x0 C.) to about 2,400xc2x0 F. (1,320xc2x0 C.) for a period of time of from about 30 minutes to about 4 days, typically about 4 hours for working embodiments.
Working embodiments of the present method typically comprised providing at least one refractory powder selected from the group consisting of materials comprising yttria, zirconia, alumina, and mixtures thereof. xe2x80x9cMaterials comprisingxe2x80x9d indicates that such materials can include other materials, including materials designed to decrease slurry aging and increase slurry lifetime, such as co-fused materials, doped materials, etc. The refractory powder was heated as described above, the temperature and period of time being sufficient to reduce the first hydration level to a second hydration level as measured by isoelectric point in an aqueous medium. A slurry was formed using the processed powder within a time period after heating, the time period being chosen such that the lifetime of the slurry is extended by at least 10% or greater, and up to at least 300% or greater, of the lifetime of a slurry formed without processing the powder according to the method of the present invention.